1. Field of the Invention
The present invention relates to an automatic frequency control (AFC) circuit for use in a receiver of radio communication or satellite communication equipment. More particularly, the present invention is concerned with an AFC circuit providing frequency stability sufficient for use in a receiver in a single channel per carrier (SCPC) system.
2. Description of the Related Art
In a SCPC system, since a bandwidth occupied by a single channel is very narrow (several dozen kHz) compared with a carrier frequency (more than several hundred MHz), and tends to be narrower every year to raise the utilization factor of frequencies, a high level of stability of a received frequency is strongly recommended to ensure stable communication.
However, the stability of a frequency of a signal received from a satellite is limited to a certain level because of unstable influences from a local oscillator mounted on the satellite, the Doppler effect, and the instability of a high-frequency circuit of a receiver.
Therefore, an AFC system is employed to ensure stable communication. In the AFC system, a pilot signal not including modulation components is transmitted in a specific frequency position within the used band, and the frequency of a local oscillator of the receiver is automatically controlled so that the frequency of a received pilot signal is converted into a precise frequency in a frequency converter.
The local oscillator consists of a voltage controlled oscillator (VCO). A received radio-frequency signal or an intermediate frequency signal is converted into an intermediate frequency signal or a lower intermediate frequency signal respectively by being mixed with an output of the VCO in the frequency converter. The pilot signal is extracted from an output of the frequency converter through a band pass filter having a narrow passband and is compared with a reference frequency output from a reference oscillator in a phase comparator. An output of the phase comparator is applied through a loop filter to the VCO as a control signal, to thereby control the frequency and the phase of the extracted pilot signal so as to be equal to those of the reference signal. Since frequencies of the other received signals within that band fluctuate simultaneously with the received pilot signal, the fluctuations of the other received signals are compensated for in converted signals.
Usually, the pilot signal is transmitted only from a master station or a reference station supervising a whole communication network. Therefore, the pilot signal is not obtained when a transmission of the pilot signal is terminated for some reason or other, when the receiver is used in a system where the pilot signal is not transmitted, or when a loop test is carried out within a part of the transmission paths.
Conventionally, in order to maintain receiver functions even when the pilot signal is not obtained, a switch is provided between the VCO and the loop filter for selecting the output of the loop filter or a fixed bias voltage. If the pilot signal is obtained, the switch is set so as to select the output of the loop filter, and if the pilot signal is not obtained, the switch is set so as to select the fixed bias voltage.
In the above construction, since frequency stability of outputs of the frequency converter is determined by the stability of the VCO while the pilot signal is not obtained, a high level of stability is required in the VCO. However, since the stability of the VCO is determined by an oscillation frequency, a variable range, and the quality factor (Q) of a resonator, etc., it is very difficult to obtain a VCO having a high level of stability as well as an ultra high operating frequency (more than several hundred MHz) and a variable range necessary for searching over the assigned frequency band to catch the pilot signal. For example, frequency stability of 10.sup.-6 is realized by using a crystal oscillator as a reference oscillator when the pilot signal is obtained, whereas the same frequency stability cannot be realized in practice when the pilot signal is not obtained, because frequency stability of the VCO is generally about 10.sup.-4. In this case, if the output frequency of the VCO is 1,120 MHz to convert 1,190 MHz into 70 MHz, the output frequency of the VCO fluctuates over a range of 112 kHz (1,120 MHz.times.10.sup.-4). Therefore, if the bandwidth of a channel is 45 kHz, a designated channel cannot be received because the width of the fluctuation is larger than the bandwidth.
Meanwhile, the band pass filter for extracting the pilot signal from the output of the frequency converter has a narrow passband such that the width of a 3 dB passband is, for example, 30 kHz in a 80 MHz band, and a crystal filter is often used as the band pass filter. Although a single frequency is usually used as the frequency of the pilot signal, a spare frequency is reserved in another frequency in some systems, and it is previously announced that the spare frequency will be used for the pilot signal when the pilot signal in the current frequency is terminated for some reason or other. In that system, if the frequency of the pilot signal is switched from current to spare, a central frequency of the band pass filter and the frequency of the reference oscillator must be changed in accordance with the changed pilot frequency.
The change may be possible if a plurality of the band pass filters and the reference oscillators having individual frequencies are provided within the receiver and each one of those is selected according to the pilot frequency. However, this solution is not economical because the band pass filters having a narrow passband, such as the crystal filters, are often expensive and the reference oscillator is also expensive.
Alternatively, the change may also be possible by exchanging the band pass filters and the reference oscillators according to the pilot frequency. However, the exchange of the filters and oscillators requires the shutdown of the system and requires excessive labor for the shutdown and exchange.
The problem of changing the pilot frequency also arises when the pilot frequency is unknown at the stage of shipping because the pilot frequency is not unique but is different if the satellite used or manner of usage thereof is different.
Furthermore, if the pilot frequency is not a typical value, an oscillator specially made to order must be used as the reference oscillator. The specially made oscillator is more expensive than typical oscillators.